The present invention relates to a communication system in which signal compression and signal expansion are required, and more particularly to a time division exchange which can compress and expand information and is suitable for use in the above communication system.
In a digital network, as described in, for example, Japanese patent application JP-A-No. 57-101,464, a signal to be transmitted is usually compressed, to utilize a transmission line efficiently. In more detail, an original signal (that is, a voice signal or a video signal) is compressed so that the information quantity of the signal becomes less than the ordinary information quantity thereof (for example, the ordinary information quantity of a voice signal having been subjected to pulse code modulation is 64 Kbps). The compressed signal is send out, and then decoded on the receiving side so as to obtain a signal which is substantially the same as the original signal. In a case where multiplex communication is made with the use of a single transmission line, the above method is very useful. In this case, however, it is required to previously assign one of the compression laws to each time slot, on the transmitting side, and to previously assign one of the expansion laws which corresponds to the compression law, to each time slot, on the receiving side. Accordingly, when the traffic expected for each compression law varies, the number of vacant time slots increases, and thus the transmission efficiency is reduced. Further, a queue is generated, and thus the communication service is degraded.
Further, as described in Japanese patent Application JP-A-No. 58-195,337, a switch is added to the above-mentioned collection/delivery apparatus, to make it possible to selectively use one of the blocks which are included in the apparatus. In this apparatus, however, not every time slot is subjected to signal compression or signal expansion. Accordingly, the above apparatus having switching means is ineffectual for variations in traffic.
That is, in these conventional techniques, no attention is paid to variations in traffic. Accordingly, in a case where a great change in traffic increases the number of calls which require specified compression and expansion laws, the transmission efficiency is reduced, and the communication service is degraded by the generation of a queue.